1. Field of the Invention
The present invention generally relates to a surface acoustic wave filter, and more particularly, to a surface acoustic wave filter that is used for high-frequency units of mobile communication devices and is required to have filter characteristics with a high shape factor. The present invention also relates to a surface acoustic wave duplexer (an antenna duplexer) having the surface acoustic wave filter.
2. Description of the Related Art
A mobile communication system today has a transmission band and a reception band very close to each other, so that the limited frequency band area can be efficiently utilized. In the PCS (Personal Communications Service) in North America, for example, the frequency gap between the transmission band and the reception band is only 20 MHz, though the pass band is located in 1.9 GHz.
To avoid crosstalk in such a mobile communication system, the frequency region (the transition region) from the pass band to a stop band needs to be very narrow in an antenna duplexer that separates transmission signals from reception signals.
To produce such antenna duplexers, dielectric filters or film bulk acoustic wave resonators (FBAR) have conventionally been used.
With a dielectric filter, however, the antenna duplexer becomes large in size. With a FBAR, on top of the problem of large size, it is necessary to control the film thickness with high precision, resulting in a poor production yield and high production costs.
In view of the above facts, more and more antenna duplexers are being produced using surface acoustic wave filters, so that smaller and thinner antenna duplexers can be achieved with a high production yield.
In general, however, a surface acoustic wave filter exhibits bandpass characteristics with a poorer shape factor than filter devices including dielectric elements or FBARs. Therefore, in order to produce a small and inexpensive antenna duplexer at a high yield, it is essential to improve the shape factor of the bandpass characteristics of each surface acoustic wave filter.
The shape factor of the bandpass characteristics needs to be improved, also because an antenna duplexer is to be located immediately before an antenna. Accordingly, each antenna duplexer needs to have a smaller loss and higher attenuation characteristics than an interstage filter.
For these reasons, there has been an urgent need to develop a surface acoustic wave filter having bandpass characteristics with an excellent shape factor.